1. Field of the Invention
The present invention relates to an optical signal receiving apparatus, and more particularly to an optical signal receiving apparatus of a multi-valued phase modulation method for modulating an optical phase in multi-values, which can be decoded into a proper electric signal array in a short time.
2. Description of the Related Art
In recent years, an access network has rapidly progressed for opticalization, and various information communication services using such large-capacity access network go on increasing. With increased demands for large-capacity data communication on both aspects of hardware and software, there is a growing request for higher bit rate of the optical fiber communication.
However, various problems arise with the higher bit rate of the optical fiber communication. First of all, with the higher bit rate of the optical fiber communication, it is required to correspondingly increase the operation rate of an electrical device and an optical device in a transmitting and receiving apparatus. At present, an Intensity Modulation-Direct Detection (IM-DD) method that is a main stream of the optical fiber communication is one of directly replacing ‘0’ and ‘1’ of an electric signal with OFF and ON of light in a transmitter and reproducing again ‘0’ and ‘1’ of the electric signal in a receiver. Accordingly, in trying to improve a signal of 10 Gbits/sec in the main stream at present for the high speed transmission of, e.g., 40 Gbits/sec, it is required that the operation speed of an optical device such as laser or photodiode, or an electric device such as an electric amplifier or discriminator for driving them is quadrupled. The electric or optical device operating at 40 Gbits/sec has a significant problem with the material or manufacturing cost, in addition to some technical problems.
Along with the higher bit rate of the optical fiber communication, an optical waveform is deteriorated due to wavelength dispersion or polarization dispersion characteristics in the optical fiber, so that a transmission speed or a transmission distance is restricted. The transmission distance with the wavelength dispersion limitation is inversely proportional to the square of transmission bandwidth, and for example, if a signal of 10 Gbits/sec is changed in speed to 40 Gbits/sec, the distance is reduced to one-sixteenth. On the other hand, the transmission distance with a polarization dispersion limitation is inversely proportional to the transmission bandwidth, and similarly, if a signal of 10 Gbits/sec is changed in speed to 40 Gbits/sec, the distance is reduced to one-fourth.
Further, the higher bit rate increases the occupied signal bandwidth. For example, in trying to quadruple the bit rate, the occupied signal bandwidth, namely, the occupied optical spectrum width is also quadrupled. In an attempt to realize large-capacity transmission by making the multi-channel of optical signals in a wavelength direction as in the wavelength division multiplexing (WDM) transmission, the bandwidth is restricted by the amplification bandwidth of a light amplifier that collectively amplifies the wavelength multiplexing signals. That is, considering that the wavelength is set so that the wavelength multiplexing signals may not overlap, it is required that the product of the occupied spectrum width and the wavelength number is in an amplification band of this light amplifier. Since the amplification band is fixed, it is required that the wavelength number is decreased if the occupied spectrum width is increased, whereby even if the bit rate is increased, the spectrum width is expanded correspondingly, and the wavelength number is decreased, and after all the total capacity is unchanged, so that it is restricted to increase the capacity.
As a method for realizing a higher bit rate by abolishing the limitation of such a device response speed, restriction due to wavelength dispersion or polarization dispersion and restriction by increased spectrum width, an optical multi-valued modulation is noteworthy. The optical multi-valued modulation is a technique for increasing the total transmission capacity log M times (the base of logarithm is 2), without increasing the bit rate of a modulation drive signal, by modulating the optical intensity or optical phase, or both in M values (M>2). More specifically, in the case of forming a signal of 40 Gbits/sec, for example, a drive signal of 40 Gbits/sec is required in the conventional binary transmission. On the other hand, the transmission capacity can be increased log 4 times=twice in quaternary transmission, and if a drive signal of 20 Gbits/sec is provided in two systems, the transmission of 40 Gbits/sec can be realized. Similarly, since the transmission capacity can be increased threefold in eight-valued transmission, the transmission capacity can be quadrupled in sixteen-valued transmission with three systems of a signal of about 13 Gbits/sec, whereby the transmission of 40 Gbits/sec can be realized with four systems of a signal of 10 Gbits/sec.
Also, in the transmission using such an optical multi-valued modulated signal, the wavelength dispersion, polarization dispersion and occupied spectrum width are restricted by the rate of these drive signals, whereby the signal of 40 Gbits/sec formed by two systems of the signal of 20 Gbits/sec can be increased fourfold in the wavelength dispersion limited distance, and twice in the polarization dispersion limited distance, and further reduced to one-half in the occupied spectrum width as compared with the signal of 40 Gbits/sec in the conventional binary transmission.
Among the optical multi-valued modulations, especially a quaternary phase modulation (DQPSK: Differential Quaternary Phase Shift Keying) in which the optical phase is modulated in four values is noteworthy, because of the advantages of the easiness of managing the interval of each level constantly and the higher sensitivity due to the phase modulation.
FIG. 1 is a configuration diagram of a quaternary phase modulation transmitter. FIG. 2 is an explanatory view of an extinction characteristic for an MZ type modulator. FIG. 3 is an explanatory view of an example of phase modulation in the MZ type modulator.
The principle of a typical method for forming a quaternary phase modulated signal will be described below using FIG. 1. A signal light outputted from a light source (1) is split into two by a band splitter (2). The two split signal lights reach the phase modulators A, B (3A, 3B). An electric signal in which a bias voltage 1A (6A) is superimposed on a data signal A (7A) by a bias superimposing unit (8A) is applied to a phase modulator A. With this electric signal, an optical signal entering the phase modulator A (3A) is subjected to binary phase modulation and outputted.
This behavior will be described below using FIGS. 2 and 3. A Mach Zehnder (MZ) interferometer is generally employed for this phase modulator. An output characteristic (extinction characteristic) of the MZ type modulator is shown in FIG. 2. If the applied voltage (transverse axis) of the MZ type modulator is changed, the output (longitudinal axis) from the modulator traces a locus similar to a sinusoidal wave as shown in FIG. 2. A voltage required for the extinction characteristic to vary from trough to crest is defined as Vπ, which is a main parameter representing the characteristic of the modulator.
Herein, a case wherein a binary electric waveform having an amplitude of 2Vπ around the trough of the extinction characteristic (in which the bias voltage is made coincident with the trough of the extinction characteristic) is applied to this modulator is considered, as shown in FIG. 3. Because of modulation from crest to crest of the extinction characteristic, the output of the modulator has a waveform of once falling from crest to trough and rising to crest again. That is, the output is always located at the crest at the center of bit, with a constant amplitude. However, there is a characteristic that the phases of the output light are different from each other by π at the adjacent crests in the extinction characteristic of the MZ type modulator. In view of this characteristic, an input electric waveform ‘0’ is converted into an output light having amplitude ‘1’ and phase ‘0’, and an input electric waveform ‘1’ is converted into an output light having amplitude ‘1’ and phase ‘π’. That is, a binary phase modulated signal having the fixed amplitude and the phase ‘1’ or ‘π’ is formed.
FIG. 4 is an explanatory view of an example of a phase state in a quaternary phase modulator.
The phase state at point A in FIG. 1, or in the output of the phase modulator A (3A), is shown in FIG. 4. Each graph of FIG. 4 represents the phase on the complex plane with the I-axis and the Q-axis, in which the I-axis indicates the amount of in-phase component and the Q-axis indicates the amount of orthogonal component. Also, if any signal point on a coordinate axis is arranged, the distance from the origin to a signal point represents the amplitude of the signal. Also, the angle between a line connecting from the origin to the signal point and a line connecting from the origin in the positive direction of the I-axis represents the phase of the signal. The phase state at point A is arranged at two points on the I-axis in symmetry of the origin. That is, the data signals having amplitude ‘0’ and ‘1’ are transformed into two points having phase ‘0’ and ‘π’, respectively.
Similarly, an electric signal in which a bias voltage 1B (6B) is superimposed on a data signal B (7B) by a bias superimposing unit (8B) is applied to a phase modulator B. With this electric signal, an optical signal entering the phase modulator B (3B) is subjected to binary phase modulation and outputted. The phase state at point B, or in the output of the phase modulator B (3B), is arranged at two points on the I-axis in symmetry of the origin, like point A, as shown in FIG. 4. That is, the data signals having amplitude ‘0’ and ‘1’ are transformed into two points having phase ‘0’ and ‘π’, respectively.
Further, a phase shifter (4) is installed at the output of one of the two phase modulators, or the phase modulator B (3B), to adjust the phase to be shifted by π/2 by applying a constant bias voltage 2. As a result, the phase states at points C and D are different from each other, as shown in FIG. 4. The phase state at point D is moved to two points arranged on the Q-axis in symmetry of the origin, because each signal point is rotated by π/2.
The output of the phase modulator A (3A) and the output of the phase shifter (4) are synthesized by a wave synthesizer (5). The output of the wave synthesizer (5), namely, the phase state of the signal point at point E is shown in FIG. 4. The signal points, at the time before wave synthesis, correspond to four points as indicated by a small circle of the broken line in FIG. 4, but as a result of wave synthesis by the wave synthesizer (5), the signal point obtained by making the electric field synthesis, or geometrical vector synthesis, of these four points is outputted. That is, assuming that the state of data A and the state of data B are represented by ‘x’ and ‘y’, respectively, the signal point when data A is ‘0’ and data B is ‘1’, namely, in the case of “01”, is arranged in the fourth quadrant of the coordinate system. Similarly, the points of “00”, “10” and “11” are arranged in the first, second and third quadrants, respectively. In this way, the quaternary phase modulated signal having four phase levels of +π/4, +3π/4, −3π/4 and −π/4 is formed.
With the above method, in the case where the signal of 40 Gbits/sec is subjected to quaternary phase modulation, two systems for the signal of 20 Gbits/sec are decomposed for the data signals X·Y, and the combinations of the data signals X·Y in these two systems are implemented by applying the phase levels corresponding to “01”, “00”, “10” and “11”. Actually, in a sending pre-coder (9), a pre-code process for converting A·B into X·Y is performed so that the detected signals in two systems may become desired A·B data in decoding in an optical receiver as will be described later.
FIG. 5 is a configuration diagram of a quaternary phase modulation receiver.
An optical receiving component for demodulating the signal subjected to quaternary phase modulation will be described below using FIG. 5. A received optical signal (10) is split at 1:2 by a band splitter (11), so that the optical signals after splitting are inputted into one-bit delay interferometers A, B (12A, 12B). Herein, each of the interferometers A, B (12A, 12B) is typically composed of an MZ interferometer. The delay interferometers A and B (12A, 12B) have an interference characteristic having a phase difference of π/2, and are employed to extract the orthogonal phase components in FIG. 4. Each of the one-bit delay interferometers A, B (12A, 12B) has a one-bit delay element, whereby each input signal interferes with the signal before one bit. For example, if the phase difference from the signal before one bit is ‘0’, the optical signal with an intensity of ‘1’ is produced by interference. On the other hand, if the phase difference from the signal before one bit is ‘π’, the optical signal with an intensity of ‘0’ is produced by interference. In this way, the phase component applied to the optical signal becomes the modulated signal with intensity ‘0’ or ‘1’ through the one-bit delay interferometer A, B (12A, 12B), and is converted into an electric signal in a balance photo-diode (13A, 13B) for the two systems at the latter stage. The signals extracted from the balance photo-diodes (13A, 13B) for the two systems through the pre-code process on the sending side as previously described are decoded as the signals A·B of 20 Gbits/sec that run one after another.
The signals converted into the electric signals by the respective balance photo-diodes are decoded in clock and data recovery (CDR) circuits (14A, 14B), multiplexed in a multiplexer (15), and decoded into a serial electric signal of 40 Gbits/sec. Then, the electric signals are parallelized in accordance with the operation speed of the latter stage circuit by a serial to parallel converter (16), and subjected to termination of an OTN frame, error correction, and extraction of a payload area (transmitted signal information) in an OTN framer (17) at the latter stage. Herein, as a method for mapping the signal information to a fixed-length frame, an Optical Transport Network (OTN) frame as defined in the ITU-T G.709 (Interfaces for the Optical Transport Network (OTN)) is exemplified, although the mapping method is not limited to the OTN frame as long as it has the fixed-length frame.
Also, as the related art, an optical signal receiving apparatus for receiving and demodulating an optical signal subjected to DQPSK modulation, comprising a parallelization section for parallelizing the multiplexed signals by inputting a DQPSK optical signal and a logical processing circuit for making a logical inversion process, a bit delay process and a bit swap process, correspondingly to a reception state at the parallelization timing in the parallelization section was described in JP-A-2007-288702 and JP-A-2008-28559. Also, ITU-T G.709 Interfaces for the Optical Transport Network (OTN) discloses a related art of the invention.